The invention relates to a large-scale autoclave for the polymerization of vinyl compounds in aqueous suspension. According to present usage, a large-scale autoclave is understood as meaning an autoclave having a volume of more than 50 m.sup.3 (13,200 gallons).
The polymerization of vinyl compounds in aqueous suspension is carried out by dispersing the monomeric vinyl compounds, which are optionally present under pressure in the liquid phase, by intensive agitation in the continuous aqueous phase, the dispersed phase being stabilized by the presence of a protective colloid. Polymerization is started by monomer-soluble initiators that decompose at an elevated temperature to form free-radicals and the resulting heat of polymerization must be removed.
Especially in the case of polymerization of vinyl chloride according to the suspension process, the trend is to use relatively large polymerization units. Thus, for example, a large-scale autoclave having a volume of 200 m.sup.3 has been described (Hydrocarbon Processing, Nov. 1976, pages 177 ff.). The use of large polymerization units has economic as well as technical advantages. One of the economic advantages is the low investment cost since only one set of autoclave auxiliary equipment, for example, vents, fixtures, lines, controls, etc., is needed in contrast to the case of a relatively large number of smaller units. Likewise, the smaller number of operating personnel required is also of economic advantage. One of the technical advantages is the improved consistency of the quality of the products and another is, for example, the lower speck-content of the polymers.
Because of the relatively unfavorable surface-to-volume ratio, the removal of the heat of polymerization in the case of large-scale autoclaves is less efficient. It has generally been observed that the removal of the heat of polymerization through the autoclave wall alone is not sufficient, especially if production conditions are to be economical, that is high polymerization rates and high spatial capacity, with a high monomer:water ratio.
The use of a reflux condenser may be helpful in this case. In the case of polymerization batches that form a large amount of foam, however, the condenser becomes coated very rapidly and becomes ineffective as a result. If the danger of insufficient heat removal is to be avoided in this case, the condenser must be cleaned frequently during the course of repeated polymerizations. The condensate flowing back into the autoclave can also lead to problems (Chemtech. May 73, page 308). Finally, when operating a completely full autoclave, the use of a reflux condenser is not possible.
The final paragraph of page 2 of German published application DE-OS No. 2,032,700 indicates that, in large-scale autoclaves, wall-cooling alone is not sufficient to remove the heat of polymerization. It is also stated that agitation in narrow autoclaves is not sufficiently effective in all parts of the autoclave.